If you're getting ready to sing, a 20-minute warm-up routine should be enough. You can always practice vocal exercises for up to an hour to strengthen the strings and extend your range on less busy days. If you want to make changes to your voice in a few months, practice as often as possible. If you can only have quality time twice a week for serious vocal time, do it, but try to increase the frequency as you can.
Not only do their voices get tired faster, but vocal concepts are newer and therefore more difficult to understand Practice little and often, set goals and keep a diary or record your voice so you can see how much you improve. And of course, practicing singing will help you maintain your vocal health, but you have to warm up your vocal cords and not overdo it, anything more than an hour of singing at a time is probably too much for your voice. If your only goal to practice is to sing until an alarm goes off, then you will achieve relatively little compared to goal-oriented practice sessions. Once your voice is stronger and more agile, you should be able to adapt your practice sessions to train your voice to do things you weren't able to do before.
You should identify what you want to improve and focus on that aspect along with your general vocal exercises. Singing when you are sick can damage the vocal cords and it is possible to practice too much and damage the voice. If you have a singing teacher, they'll give you advice on songs to practice with or areas to focus on, and it's always worth talking to other singers and asking what practice songs they use. If you record yourself or keep a singing practice diary, you will see the improvement and you will also monitor your vocal health.
When all is said and done, no one can predict how long it will take you to achieve your vocal goals. You know from the moment you set the timer to the moment it goes off, that the only worry and focus you have is singing, achieving your vocal goals and improving. Not only will your voice get tired faster, but vocal concepts will be newer and harder to understand. You can then focus on those areas during your practice sessions, this will become more challenging and also rewarding as your voice improves.
Regular and varied practice will develop your technique, develop your vocal cords and help you train your voice.